Flood is probably the greatest devastating, wide spread and frequently natural hazard of the world wide that producing several socioeconomic and environmental consequences within the affected floodplain. In Ethiopia, Awash River basin is one of the areas affecting by flood plain problem and among Awash River basin, wonji to awash melkessa is one the most frequently severed area by floodplain along Awash River, for those flood risk analysis is so important. The main objective of this study was flood inundation mapping and quantifies the risk of flood on the area by using hydrological model HEC-HMS for peak flood forecasting for different return period and GIS extension HEC-GeoRAS integration with hydraulic model HEC-RAS for flood inundation mapping. For hydrological model (HEC-HMS) daily rainfall and stream flow data of 16 years from (1998-2013 G.C) was used. Among the data used eight years for calibration and five years for validation of HEC-HMS model at wonji Gauged station. HEC-RAS model was calibrated and validated using satellite imaginary Landsat-7 flood event and observed flood event of September, 06/2000 and September, 22/2007 G.C respectively. The missing values were filled by using normal ration method for daily precipitation data. Consistent data was checked by double mass curve method. The initial and constant loss method was selected for loss method and for transform method Clark unit hydrography was applied. The estimated peak flood by hydrological model (HEC-HMS) was 147.2m3/s, 161.9m3/s, 329.4m3/s, 409.5m3/s and 506.8m3/s for 5year, 10year, 25years, 50years and 100year return period correspondingly. The flood inundated area corresponding to peak flood were 1052.7 hectare, 1086.9 hectare, 1429.5 hectare, 1559.1 hectare and 1694.1 hectare respectively. The total crop losses were calculated for flood inundated area, for 5 years and 100 year return period of crop loss were 75501.6 quintal and 121503 quintal correspondingly. Thus finding of the study may help in planning and management of flood plain area to mitigate probable disaster through technical approach.